Long story short: I have tried to stabilize my Gentoo installation for almost 2 months and no success. Until now I thought that this is problem in Gentoo kernel (tried: 3.8.13, 3.10.7, 3.10.10, 3.11.0, 3.11.1) as I was unable to reproduce unstability on any other installation I tried - Win7, ubuntu, kubuntu, mint, fedora, estobuntu. I tried to remove all hard disks, removed nvidia videocard and pci ethernet card. ran memtest for day.. With live OS'es nothing, but Gentoo was crashing (and before os crash sometimes programs also crashed, like gkrellm, firefox, konsole, ..) randomly as sometimes just GUI halted, sometimes keyboard lights were blinking and sometimes I got some randome kernel traces also on screen.

But now, after 1d long running Estobuntu (almost all time some movie was running, shared some ubuntu iso over bittorrent, etc - computer was doing smth all the time) I got some new bits - in dmesg were

I don't think anymore, that it is purely Gentoo's problem, as this mcelog in my first post was produced under Estobuntu (Estonian version of Ubuntu) AND Gentoo livecd (20121221) was also unstable - three hangs in 24 hours. Just Gentoo is somehow more intense or active in some areas and crashes may happen in 30minutes.

The OP's current CFLAGS and CXXFLAGS are reasonable. Adding -march=native might improve performance in some cases, but will not correct problems caused by failing hardware. Suggesting that he switch to newer packages is also not helpful. According to the mcelog output, there is a hardware fault. The particular error claims to have been corrected, but there may be related errors that are not correctable. The faulty component must be replaced.

This -march flag should make no difference, as this is default. And problem is not anymore only Gentoos as I was able to produce problems on other distro too. Question is - what does this mcelog mean? As I quess, this is some weird hardware problem?

I add also three images of traces I have been able to capture as computer hung up (and as seen - they are random and in pretty "weird" places):

As MCELOG says, this is a hardware error.
Check your chipset to make sure it's not overheating too. Sometimes I wonder about Gigabyte boards, they don't have fans on their chipsets but that heatsink gets quite hot. (I have a Gigabyte Z68AP-D3 and EP43-UD3L boards, neither have fans on the chipset)
Checking with RAM chips removed was a good idea.

Since you have a K-series chip, try to underclock to see if it helps, especially try to see what it does if you reduce BCLK from 100MHz. Also possibly increasing DRAM and/or chipset voltage.

It's weird that an ubuntu doesn't work, though you should try a stock ubuntu if you can. Their optimizations tend to allow any CPU to work._________________Intel Core i7 2700K@ 4.1GHz/HD3000 graphics/8GB DDR3/180GB SSDWhat am I supposed to be advocating?

I'm having the same errors as previous poster with a Xeon E3-1275v3 (Haswell) but it only happens when I'm running Virtualbox with IO-APIC enabled with FreeBSD/OpenBSD while compiling ports, if I disable IO-APIC, no errors. It also happen under VMware player with FreeBSD. I have yet to see those errors while not running a VM and it doesn't happen while running a Linux VM, for example, I have emerge world on Gentoo VM over 400 packages without a single error and I did it twice to be sure. After weeks of uptime, no MCE event of this kind if not in a VM context, memtest and prime95 torture tests runs without errors and I compiled various things on the Linux host, no errors. I'm using Kernel 3.12.

I'm also seeing machine check events when running a Windows SBS 2003 32 bit virtual machine with qemu on my Xeon E3 Haswell system. I found a thread about this on the vmware forums: https://communities.vmware.com/thread/452344 – but even after changing the qemu machine, I only got less machine check events, they were not gone.

Just got call from warranty repairs.. motherboard was broken, changed to MSI. Gotta see, how it works
Thanks of every bit of advice.

The same happened to me on a K8WE tyan board with dual opteron 280 and 8x1Gb PC3200 ECC/REG. Was surfing when the systam locked up. Rebooted and was greted by a kernel crash saying it was a hardware error and no software error. After some testing I discovered that one memory channel on the board had broken
Removed both dimms in that channel and the kernel booted. If I added it back i got the same errors you had. Got my hands on a second K8WE board and both CPU and RAM worked in that. Tough that board died after the capacitors started leaking when I had it in storage so in the end I never realy got around to butting it back together other then for testing. Ended up getting new components._________________WS: i7 3930K @4Ghz, 32Gb ram, 128Gb SSD, GTX780 3Gb, NICs: Intel 2x1Gbit
NAS: Core I3-4360 3.7Ghz, 20Gb ram, 256Gb SSD, 35Tb HDD, NICs: Intel 2x1Gbit
ROUTER: Celeron J1900 2Ghz, 8Gb ram, 128Gb SSD, NICs: 2x1Gbit, WIFI: Intel AC 7260, Atheros AR5005G

HSD131. Spurious Corrected Errors May be Reported
Problem: Due this erratum, spurious corrected errors may be logged in the IA32_MC0_STATUS
register with the valid field (bit 63) set, the uncorrected error field (bit 61) not set, a
Model Specific Error Code (bits [31:16]) of 0x000F, and an MCA Error Code (bits
[15:0]) of 0x0005. If CMCI is enabled, these spurious corrected errors also signal
interrupts.
Implication: When this erratum occurs, software may see corrected errors that are benign. These
corrected errors may be safely ignored.
Workaround: None identified.
Status: For the steppings affected, see the Summary Table of Changes.